1. Field of the Invention
The present invention relates to an injection molding machine, and more particularly, relates to an injection molding machine that controls drive of a movable part with a motor.
2. Description of the Related Art
An injection molding machine is provided with many motors. These motors include an injection motor for injection operation in the injection machine, a screw rotation motor for rotating a screw for sending resin in the injection machine, a mold opening/closing motor for opening, closing, and clamping a mold, and an ejection motor for ejecting a molded article. To increase the quality of the molded article in the injection molding machine, it is important to perform feedback control of the control conditions of the injection speed and injection pressure. For the feedback control, the speed of each motor is to be controlled.
Generally, when the same torque is applied, a servomotor used in the injection molding machine has a higher duty as the capacity of the motor increases. If the high duty motor is used, maximum torque of the motor may exceed an allowable value of the mechanism section of the injection molding machine body. In such a case, the mechanism section of the injection molding machine body may be affected. To solve the problem, an upper limit (torque limit) is set to the motor output torque to protect the mold and the mechanism section of the injection molding machine.
If the motor rotates at a constant speed, or the motor rotation speed is almost zero and only torque is produced, the load applied to the mechanism section is expressed in a relationship of a linear function of the torque output by the motor, and limited to a constant value by a torque limit. However, by the method of setting the torque limit of the constant value, the limit is inevitably set despite the need of torque at acceleration or deceleration, and in some cases, the motor has to operate with slow acceleration or deceleration.
Japanese Unexamined Patent Application Publication No. 6-246801 discloses a method of driving an injection screw with a servo motor to move the screw forward for injection. The injection molding machine includes a torque limiting means that can control the torque of the servo motor. In an adjustable speed area, during primary injection processing, the torque limit for the servo motor by the torque limiting means is released. In other areas than the adjustable speed area, during the primary injection processing, the torque limit is applied by the torque limiting means.
Japanese Unexamined Patent Application Publication No. 9-254220 discloses a motor-operated injection molding machine that temporarily releases a limitation of the magnitude of the output torque of an injection driver amplifier at start-up of an injection process to reduce the rise time of the injection process to increase the injection molding efficiency.
Japanese Unexamined Patent Application Publication No. 4-334429 discloses a method of starting a screw of an injection molding machine by limiting torque to initial torque that is smaller than torque (normal torque) in a steady state and then sequentially increasing the output torque from the initial torque to the normal torque to prevent an impact at the start-up of the screw to smoothly and stably start the screw to effectively prevent breakage of the screw.
According to the known methods of setting a torque limit of a constant value, however, the limit is set to the torque necessary at the acceleration or deceleration, and in some cases, the machine has to operate with slow acceleration or deceleration.
In the techniques disclosed in Japanese Unexamined Patent Application Publication No. 6-246801 and Japanese Unexamined Patent Application Publication No. 9-254220, the limitation of the torque is released at start-up of the injection process, and the like so that appropriate acceleration or deceleration can be provided at the start-up of the injection process and the like. However, since the limitation of the torque is completely released, depending on the value of the acceleration or deceleration, the maximum torque of the motor may exceed the allowable value of the mechanism section of the injection molding machine body. In such a case, the mold and the mechanism section of the injection molding machine may break.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 4-334429, the torque limitation is performed at the start-up time to prevent breakage of the screw. Accordingly, the breakage of the screw can be prevented, however, the acceleration or deceleration speed may be decreased more than the case of setting the torque limit of the constant value.